Rapid prototyping or solid free-form fabrication has become an increasingly important manufacturing tool. In the past, three dimensional contoured articles have been prepared by cutting a suitable substrate material, i.e. paper or highly layered and resin impregnated plywood, into planar shapes having an edge contour matching that of the article to be fabricated along the particular plane, and stacking and laminating these shapes into an article with a contour approaching that of the final article. Such methods have been used, for example, to prepare model ship hulls and the body components for one of a kind "concept cars". The edges are smoothed to present the final product, the degree of smoothing necessary being dependent on the thickness of each layer. Such methods are labor-intensive and suitable only for relatively large parts.
In the mid-1980's, an automated process for preparing three dimensional articles termed "stereolithography", was developed, as evidenced by U.S. Pat. No. 4,575,330. In the stereolithographic process, hereinafter "SLA", a substrate is immersed in a photocurable resin to a predetermined, shallow depth, and scanned with a highly focused or collimated ultraviolet laser beam. The laser scan is computer controlled, with the scanning parameters derived from a CAD file corresponding to a section in plane of the proposed prototype shape. The photocurable resin polymerizes to a solid plastic material where struck by the laser beam, forming a single layer having a thickness in the range of 100-200 .mu.m. The substrate is then lowered in the resin bath forming an additional polymerizable layer which is in turn scanned by the laser with a pattern corresponding to the new layer's shape as calculated from the CAD file. Prior to the second or subsequent laser scan, a wiping blade is often passed along the uppermost surface to ensure uniform resin layer depth. By repeating this process many times, a plastic article having the dimensions of the prototype part is produced.
The plastic parts produced by stereolithography may be the final part to be used in one of a kind production or small production runs, but is often the corresponding negative of such a part, which may then be used to prepare wax patterns for investment casting by the "lost wax" method. In the past, the complex molds required to prepare the wax patterns have necessitated expensive machining steps. Following preparation of the wax patterns, these are surrounded by a shell mold of foundry sand, ceramic, plaster, or the like. The mold is heated to allow the wax to melt and drain from the mold, and molten metal poured into the cavity. At times, the hot metal may be poured without removing the wax pattern, the hot metal melting the wax and displacing it from the mold. Use of SLA to prepare the mold for the wax pattern enables rapid and economic production of highly detailed investment castings.
Many investment cast articles, however, are hollow, and require casting the metal around a ceramic core. In conventional technology, the ceramic core is injection molded from a silica-based ceramic and fired. The ceramic core is inserted into the wax pattern and the metal is cast. The ceramic core is then removed by chemical leaching, leaving a hollow interior with the precise shape of the core. The core itself is often of a complex shape, for example the cooling passages in jet turbine blades, and the molds required to produce the green ceramic core are time consuming and expensive to machine. For these reasons, core design is often simplified from what is desirable from a technological standpoint, with compromises in product performance.
Ceramic cores have been prepared by molding highly loaded ceramic dispersions in a thermocurable resin, as disclosed in U.S. Pat. Nos. 4,894,194, 5,028,362, and 5,145,908. Such processes allow the use of less expensive composite, plastic, or silicone rubber molds while also providing a green ceramic body having suitable strength for handling prior to firing. However, a mold of suitable shape must still be provided, and the process does not lend itself to small production runs or single prototypes.
It would be desirable to be able to utilize SLA for the rapid prototyping or production of ceramic cores and other ceramic and sinterable metal parts. Heretofore, this has not been possible. The resins useful in SLA must have a relatively low viscosity, generally below 3000 mPa.multidot.s, to allow for recoating of the part for successive laser scanning. As explained below, for successful use of ceramics in SLA, the solids loading must be high, yet the resin must be stable with regard to sedimentation as well as presenting essentially Newtonian behavior, i.e., the resin must not be thixotropic, and must be able to flow even under low shear conditions. Such resins have not previously been available.
It would be further desirable to utilize SLA for the direct production of ceramic molds for metal casting. The requirements for ceramic SLA resins suitable for preparing ceramic casting molds are similar to the requirements for preparing ceramic cores by SLA, using instead refractory ceramic powders of composition and particle sizes known in the art of metal casting as being advantageous for the production of ceramic casting molds. Use of such processes, which may be denominated a "direct shell process" enables the production of more sophisticated structures in a ceramic mold, to control, for example, the heat flow in various parts of the mold, or the mechanical strength thereof.
Photocurable resins containing ceramic pigments have been used as ultraviolet curable coatings, for example, titanium dioxide pigmented UV-curable paints. However, the particle loading is far too low to produce a useable ceramic material. During or prior to the firing of the green ceramic body, the organic resin incorporated with the ceramic precursor particles must volatilize or decompose, a stage termed "burn-out". Resins which volatilize effectively without leaving substantial carboniferous residue but are present in substantial amounts provide porous ceramics which exhibit considerable shrinkage, often in a non-uniform fashion. Resins which produce large amounts of carboniferous substances prevent efficient sintering across grain boundaries in the ceramic, producing a ceramic part with insufficient strength. When ceramic parts are required with high dielectric constant, the carbon remaining may render the part electrically conductive. When sinterable metal particles are utilized, the carbon may dissolve into the particles or carburize their surface, markedly changing the physical properties of the finished, sintered part.
Modestly loaded, photocurable metal particle and ceramic particle pastes have been utilized to prepare microelectronic devices such as thin film capacitors, as disclosed in U.S. Pat. No. 4,828,961. However, even with particle loadings in the range of 20-43 volume percent, the resins are highly viscous pastes requiring doctor blade coating. Such pastes are not suitable for SLA, nor is their particulate loading, even at their paste-like viscosity, sufficient to prepare useful sinterable metal or ceramic parts which can be fired without exhibiting shrinkage and while maintaining acceptable physical properties.